Immunohistochemical detection of capsaicin receptors in taste cells of human circumvallate papillae

Regional brain morphology of the primary somatosensory cortex correlates with spicy food consumption and capsaicin sensitivity

Objective: Habitual spicy food consumption leads to altered perception of capsaicin. Little is known about the neural morphological correlates of habitual spicy food intake and related trigeminal perceptions. In this study, we used voxel-based morphometry to identify brain regions where regional gray matter volume (GMV) correlates to spicy food consumption. Methods: Fifty-two participants were surveyed for their spicy food dietary habit by a composite score of spicy diet duration, frequency of spicy food consumption, and preferred degree of spiciness. Forty-two participants were further assessed for oral sensitivity and intensity ratings of capsaicin-induced irritation, and intranasal sensitivity and intensity of trigeminal odors. Results: We found that the composite spicy score was positively correlated to GMV of the primary somatosensory area (SI), and the primary (M1), supplementary motor areas (SMA) and the putamen. It was negatively correlated to GMV of the anterior insula, orbitofrontal cortex, frontal gyrus and angular gyrus. The GMV of the SI area was negatively correlated to capsaicin sensitivity; the GMV of the right middle frontal gyrus was positively correlated to the irritative intensity for capsaicin at high concentration (70 μM). However, we observed no correlation between the intranasal trigeminal sensitivity and spicy food consumption or the regional GMV. Discussion: Collectively our findings suggest a central neuroanatomical reflection of altered capsaicin perception in relation to habitual spicy food consumption. Future longitudinal studies should elucidate the possible causal relationship of dietary habit and brain structural plasticity.

Peri-threshold Trigeminal Stimulation with Capsaicin Increases Taste Sensitivity in Humans

Introduction

Taste perception is affected by trigeminal stimuli, i.e., capsaicin. This has been studied at suprathreshold concentrations. However, little is known about taste perception at threshold level in the presence of low concentration of capsaicin. The aim of the study was to explore whether taste sensitivity for sweet, sour, salt, bitter, and umami is modulated by the presence of capsaicin in the peri-threshold range.

Methods

Fifty-seven adults (age range 19–85 years; 32 women) with functional gustation participated in the study. Based on their perception of phenylthiocarbamide (PTC), the group was stratified into non-tasters (n = 20) and tasters (n = 37). Threshold for sweet (sucrose), sour (citric acid), salty (sodium chloride), bitter (quinine-hydrochloride), and umami (sodium-glutamate) tastes was estimated using a single-staircase paradigm (3-alternative forced choice; volume per trial 0.1 ml) with or without 0.9-µM capsaicin added. This capsaicin concentration had been determined in pilot studies to be in the range of oral perception thresholds.

Results

The addition of capsaicin produced lower taste thresholds for sweet, sour, salty, and bitter but not for umami. In contrast, neither PTC taster status nor sex affected these results.

Conclusion

The current results indicate that a low concentration of capsaicin increases gustatory sensitivity.

Implications

The current findings provide evidence supporting different effects of capsaicin on taste perception at threshold level. It has implications for boosting taste sensitivity or flavor enjoyment with low concentration of capsaicin.

Anxiolytic efficacy of repeated oral capsaicin in rats with partial aberration of oral sensory relay to brain

OBJECTIVE

This study was conducted to examine if taste over load with oral capsaicin improves the adverse behavioural effects induced by partial aberration of oral sensory relays to brain with bilateral transections of the lingual and chorda tympani nerves.

DESIGN

Male Sprague-Dawley rats received daily 1 ml of 0.02% capsaicin or water drop by drop into the oral cavity following the bilateral transections of the lingual and chorda tympani nerves. Rats were subjected to ambulatory activity, elevated plus maze and forced swim tests after 11th, 14th and 17th daily administration of capsaicin or water, respectively. The basal and stress-induced plasma corticosterone levels were examined after the end of behavioural tests.

RESULTS

Ambulatory counts, distance travelled, centre zone activities and rearing were increased, and rostral grooming decreased, during the activity test in capsaicin treated rats. Behavioural scores of capsaicin rats during elevated plus maze test did not differ from control rats. Immobility during the swim test was decreased in capsaicin rats with near significance (P = 0.0547). Repeated oral capsaicin increased both the basal level and stress-induced elevation of plasma corticosterone in rats with bilateral transections of the lingual and chorda tympani nerves.

DISCUSSION

It is concluded that repeated oral administration of capsaicin reduces anxiety-like behaviours in rats that received bilateral transections of the lingual and chorda tympani nerves, and that the increased corticosterone response, possibly modulating the hippocampal neural plasticity, may be implicated in the anxiolytic efficacy of oral capsaicin.